1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that inverts a polarity of an image signal to be supplied to an image signal line within a certain frame.
2. Description of the Related Art
The liquid crystal display device controls gray level of a pixel due to an electric field generated between a pixel electrode included in each pixel circuit and a counter electrode. Also, a voltage of the pixel electrode for displaying a certain gray level has two types, a voltage of a positive polarity and a voltage of a negative polarity. In this case, the voltage of the positive polarity is a voltage higher than a voltage across the counter electrode, and the voltage of the negative polarity is a voltage lower than the voltage across the counter electrode. In this case, there has been known that when a voltage of a certain polarity is applied to liquid crystal used in the liquid crystal display device for a period longer than a given period, a relationship between the voltage across the pixel electrode and the gray level is damaged (ghost image phenomenon). Therefore, a polarity of the voltage applied to the pixel electrode is reversed for each frame.
Also, there has been known that it is difficult to prevent the ghost image phenomenon by merely reversing the polarity of the voltage for each frame. For example, when vertical lines aligned every other dot in a horizontal direction are scrolled at a certain speed in a horizontal direction, an average of the voltages to be applied to a pixel electrode of a certain pixel circuit is different from a voltage across the counter electrode. As a result, the ghost image phenomenon may occur. In order to cope with this phenomenon, JP 2007-225861 A discloses that the polarity of the voltage to be applied to each pixel circuit is further inverted every time a period corresponding to a plurality of frames elapses.